And there's a Linux version with GUI add well, check it out: f.lux for Linux. This is great especially for people working on a computer and suffering from insomnia or e.g. if you have a work room without windows (glass filled frames that is).

E-Ink readers like the Nook touch currently come with 4 bit (16 levels) of gray, no camera, often no sound, no GSM, often no landscape detection sensor etc. And especially, the display refresh rate is very slow. Still or nonetheless they have advantages like battery time, especially with the display on, clear readability in bright direct sunlight over a tablet and display size over a phone and also more and more often come with Android. Here are some apps and hints which work great for that scenario.